Systems and methods for identifying the position of cellular telephones are known in the prior art.
For example, International Patent Publication No. WO0018148 describes a method for locating cellular telephones.
The prior art method makes it possible to locate cellular telephones by comparing the Radio Frequency information collected from the cellular telephone (cellular telephone's RF fingerprint or RF measurements) with RF information contained in a reference database (reference RF fingerprint), in which each reference RF fingerprint corresponds on a one-to-one basis with an elementary area or pixel of the geographical area served by the network, and by assigning the cellular telephone to the position corresponding to the reference RF fingerprint whose values are closest to those of the cellular telephone.
While the prior art method would appear to provide reliable results from the technical standpoint, from the practical standpoint the results that this method furnishes are in general unreliable or inaccurate at best.
One practical problem is associated with the limited number of frequency channels (channels) that cellular telephones use in order to communicate.
As the number of channels is limited (In Italy, for example, GSM (Global System for Mobile Communications) networks provide 124 channels, only a portion of which are allocated to each operator, it is usual for an operator to assign identical channels to radio base stations that “cover” a given geographical area.
Because of this practice, which is also called frequency reuse, the cellular phone receives RF signals for each channel which correspond to the algebraic sum, in terms of power, of the field values received from the respective radio base stations using the same channel. Consequently, the cellular telephone's RF fingerprint for a given channel may be the same even when it is generated in different parts of the geographical area, given that it derives from a sum. As can readily be understood, this problem would not exist if the channels were not reused. In such a case, in fact, the field value for each channel would depend only on electromagnetic loss factors due to the route taken to reach the cellular telephone.
Another practical problem springs from the fact that the cellular telephones generate an RF fingerprint which includes the field values of a limited number of channels. In the case of GSM, for example, the cellular telephones are capable of generating an RF fingerprint that includes a maximum of seven values (value septuplet), each corresponding to a field RF value for a different channel.
Consequently, it is extremely probable in practice that cellular telephones positioned at different elements or pixels in the geographical area generate identical septuplets, precisely because of the limited number of values that can be used. Naturally, when the cellular telephones can provide a number of values equal to the number of available channels, the potential for error would be very limited.
A third technical problem is associated with the fact that the reference databases, even if updated regularly, contain field values which, regardless of the method used to obtain them, cannot correspond exactly to the cellular telephones' field values because of the continual variations in environmental and/or weather conditions which determine the latter values.
For this reason, it cannot be realistically assumed in practice that the cellular telephone's position is uniquely identified by a single reference fingerprint, except in particularly fortunate circumstances.
Essentially, then, though the prior art method may be accurate in theory, it has been found in practice that this method results in location errors which in many cases may be significant because of frequency reuse, the limited number of values that the cellular telephone uses to generate the RF fingerprint, and the fact that the values measured by the cellular telephone vary over time.
In GSM networks, for example, it has been determined experimentally that there is a systematical ambiguity in establishing cellular telephone location because the prior art method assigns the same septuplet to points that are geographically quite distant, and may be up to several kilometers apart.